In a motor system, it is often necessary to monitor the torque produced by the motor, particularly if the motor is being used to drive a complex load, such as, for example, a pump or a valve. Electromechanical torque transducer systems are sometimes employed, but are limited due to physical constraints, high costs, and lack of reliability and robustness. A recent alternative for monitoring torque relies on the current sensed by a single current transformer in one of three phases supplying the drive motor. In particular, the current is assumed to represent a scaled version of the actual torque produced by the motor.
Unfortunately, however, as derived from the steady-state equivalent circuit of an induction motor, the relationship between current and output torque (or shaft torque) is not fixed, but varies as both power factor and efficiency change at different loads and speeds, as shown in FIG. 1. Other factors which affect the relationship between current and shaft torque include rotor electrical and mechanical time constants and saturation. Therefore, using current to deduce the state of the load and the nature of faults results in significant errors.
Another method for monitoring torque output in an induction motor involves a model-based approach wherein knowledge of motor parameters (e.g., rotor and stator resistances, leakage inductances and magnetizing inductance) and motor speed is required in addition to motor currents and voltages. Unfortunately, however, motor parameters must be given, measured or estimated; and estimation under transient conditions is difficult and may involve complex computations.
Accordingly, it is desirable to provide means for precisely estimating torque in an induction motor using both current and voltage information, which information can be provided from circuit breaker sensors that function to protect the motor from overcurrents and undervoltages. Similarly, such information can be provided from contactor sensors or similar devices in a motor control center. Furthermore, it is desirable that such torque measurements be made without requiring measurements of motor parameters or speed.